Optical Emission in the Beam of Stellar Jets: A Possible Mechanism

We propose a mechanism for the optical emission observed in the beam section of stellar jets based on the stability properties of circularly polarized Alfvén waves propagating in the partially ionized medium filling the jet's channel. We first derive the relevant magnetohydrodynamic equations, which include the Hall term for such a partially ionized medium, a term which give rise to dispersive effects. Quasi-equipartition of thermal and magnetic energies is assumed, and the model is developed in the one-dimensional approximation but keeping the three components of the vectorial fields. Mild compressions or very weak shocks occur in the flow when instabilities develop, depending on the relative sign of the angular momentum of the underlying accretion disk and the external magnetic field. Simulations performed under quite general conditions show that the temperature/density perturbations associated with the instability are consistent with the average luminosity contrast observed in the jet's beam between bright knots and interknot regions.